It has already been suggested to stabilize the wavelength of laser diodes by stabilizing their operating parameters and by additionally tying to a wavelength reference such as a Fabry-Perot interferometer. A suitable stabilization is the precondition that laser diodes maintain the wavelength stability of 10.sup.-7 or better required for many applications such as in communications technology or in metrology.
The current and the temperature of the laser diode have a relatively great influence on the emitted wavelength and this current and temperature are, as a rule, held as constant as possible to achieve the stabilization. In addition to the stabilization of these parameters, a part of the radiation emitted by the laser diode is supplied to a Fabry-Perot interferometer with a detector being connected downstream thereof which measures the intensity passing through the Fabry-Perot interferometer. Mostly, the current and therefore the wavelength of the laser diode is modulated and the signal of the receiver downstream of the Fabry-Perot interferometer is detected with a lock-in amplifier which measures the component of the receiver signal which oscillates at the modulation frequency. The output signal of the lock-in amplifier is then a measure for the deviation of the center wavelength of the laser diode from the central wavelength of the transmission maximum of the Fabry-Perot interferometer and can be used for readjusting the diode current and therefore the wavelength which the laser diode emits. In this way, the wavelength of the laser diode can be stabilized much better than only with a so-called parameter stabilization holding the current and temperature constant. Such an arrangement is shown, for example, in published German patent application DE 35 42 090.
For a reliable operation of such an arrangement, it is however necessary that the transmission maximum of the Fabry-Perot interferometer, onto which the control circuit locks in the wavelength of the diode laser, lies in a stable range within a mode of the laser diode and that, during the operation of the laser diode, no mode jumps occur. For this purpose, an electronic circuit is provided in the above-mentioned German patent application which automatically selects a suitable transmission maximum of the Fabry-Perot interferometer to which it is intended to stabilize. For this purpose, the current and temperature of the laser diode are first varied in order to pass through a region free of mode jumps. Then, when a transmission maximum of the Fabry-Perot interferometer is detected in this region, the wavelength emitted by the laser diode is locked onto this transmission maximum.
This known arrangement is well suited for applications such as in communications technology wherein the wavelength is stabilized with high accuracy but most importantly with respect to short term fluctuations, that is, relative wavelength changes. This arrangement is not suitable for applications in distance measurement technology since it is there necessary to stabilize the wavelength of the laser diode to the same precisely defined wavelength again and again. With reference to the above-mentioned arrangement, this leads to the requirement that the stabilization always and repeatedly locks into the same transmission maximum of the Fabry-Perot interferometer since the position of this transmission maximum determines the wavelength emitted by the laser diode and therefore also the reference wavelength with which the interferometer used for distance measurement operates.
For this last mentioned application, the paper of F. Crosdale et al entitled "Wavelength Control of a Diode Laser for Distance Measuring Interferometry" published in the proceedings of the SPIE, Vol. 1219, "Laser-Diode Technology L. and Applications II" (1990), pages 490 to 503, describes a stabilization arrangement wherein the wavelength of the laser diode is stabilized absolutely at 10.sup.-6 F. Crosdale et al utilize for this purpose a Fabry-Perot etalon which furthermore has a temperature stabilized to 0.01.degree. C. in addition to the temperature stabilization of the laser diode. Under these conditions and because of the small thickness of 3 mm of the etalon, only very few transmission maxima of the etalon lie in the region of a selected mode in which the laser diode emits, it can be reasonably ensured that the same transmission maximum of the etalon is always approached and therefore the stabilization takes place to an absolute wavelength.
However, the foregoing is no longer the case where a so-called confocal Fabry-Perot interferometer is intended to be used for stabilization. Such confocal Fabry-Perot interferometers are insensitive with respect to adjustment and the position of the transmission maximum is not dependent upon the angle with respect to the beam axis as with the etalon. Such confocal Fabry-Perot interferometers have as a rule dimensions which are very much larger than with etalons. Correspondingly, at a center wavelength of approximately 830 nm, very many transmission maxima occur in the region of a mode of the laser diode. Within a mode, the wavelength can be changed with current and temperature. The coverable range is 0.5 nm with regions of typically 5 mA and 2 K in magnitude and gradients of 6.times.10.sup.-3 nm/mA or 6.times.10.sup.-2 nm/K. When the Fabry-Perot interferometer has a length of approximately 30 mm, then there are approximately 20 transmission maxima within a mode of which the correct one must be selected, when, after each switch-on and switch-off of the laser diode, always exactly the same wavelength is to be absolutely adjusted.